Why are there two tides per day?

The simplest picture is that there are two tidal bulges on Earth, and as Earth rotates you typically pass under both—one high tide on the side facing the Moon and one on the far side.

Here’s why. The Earth and Moon don’t orbit with Earth fixed in place; they both orbit their shared balance point (the barycenter), which lies inside Earth—about three-quarters of the way from Earth’s center toward the Moon. Because Earth is continuously “turning around” that barycenter, every point on Earth experiences the same outward inertial (often called “centrifugal”) effect. Meanwhile, the Moon’s gravity is not the same everywhere: it’s strongest on the near side and weakest on the far side.

The tides come from this difference (the gradient) in the Moon’s pull across Earth. On the near side, lunar gravity wins, pulling ocean water toward the Moon. On the far side, the Moon’s pull is weakest, so the outward inertial effect dominates relative to the weaker lunar pull, producing a second bulge on the far side. Those two bulges are why many coastlines experience two high tides most days.

Why are the tides 12 hours and 25 minutes apart?

If the Moon were fixed in the sky, you’d pass the two bulges every 24 hours, giving ~12 hours between high tides. But the Moon moves eastward in its orbit while Earth rotates in the same direction. So Earth has to rotate a bit more than 360° for a given location to line up with the Moon again.

The result is a lunar day of about 24 hours 50 minutes (time from one Moon-overhead to the next). Since there are two tidal bulges, you typically get two highs per lunar day, so the time between highs is about:

(Real coastlines can differ because basins, friction, and resonance distort the timing and size of the bulges, but this is the core astronomical reason for the ~12h 25m spacing.)

How does that relate to the Moon’s full orbit?

This is the nerdy (but clean) way to see where the extra ~50 minutes comes from. A sidereal month is the Moon’s orbital period relative to the distant stars: about 27.32 days. That means the Moon moves about:

After Earth rotates once relative to the Sun (about 24 hours), the Moon has moved eastward in its orbit. Earth must rotate an extra 13.2° for your location to realign with the Moon. Since 360° corresponds to 24 hours, that extra rotation takes:

Therefore the lunar day is about:

And the time between successive high tides is approximately:

Note on terminology: the synodic month (about 29.53 days) is the time from new Moon to new Moon (relative to the Sun). That’s important for phases and spring/neap cycles, but the daily tide timing hinges on the Moon’s motion relative to Earth’s rotation, which is why the sidereal month is the cleaner tool here.